1. Field of the Invention
This invention relates to cylindrical bright light sources for scanning and inspecting rapidly moving webs for surface defects therein.
2. Description of the Prior Art
Web inspection for the detection and classification of imperfections in the surface source through moving webs is an area of growing industrial interest. Such imperfections include stains and absorbing imperfections, abrasions and scattering imperfections, pinch marks, impurities preventing local processing, tears, through-holes, thickness imperfections, and other far side and near side imperfections that may print through to the surface of the thin (e.g. 0.002-0.020") webs usually being inspected. Such webs are typically inspected by a camera viewing through a lens subtending a small angle at its observing location. Such inspection conditions make it possible to detect differences in the light intensity detected by the camera due to scattering imperfections on or in the web as it is moved rapidly past the camera. Thus, line scan cameras or detectors are positioned with respect to the normal surface deflection angle to record the incidences of improper angular reflection in relation to the actual position of the web at the time. The actual position of defects may be recorded for slower speed examination or for other quality control purposes.
In my U.S. Pat. No. 4,914,308, one current system for scanning fast moving webs of paper, steel and aluminum sheet or transparent film substrates, employs high intensity, incandescent lamps having an elongated slit aperture or window with an elongated bar-shaped lens is employed to direct a strip of light across a fast moving web. The light is reflected or transmitted by the web toward a video camera or Charge Coupled Device (CCD) array which detects the intensity of the transmitted or reflected light for display on a monitor and for recording. Due to the high speed switching capabilities of full field video cameras and CCD linear arrays, is possible to derive closely-spaced lines of data while the web is moving at very high speeds. Each CCD detector site views a corresponding area along the transverse line of the moving web illuminated by bright light focused on it.
As described more completely in U.S. Pat. Nos. 4,914,308 and 4,752,897, both incorporated herein by reference in their entireties, the resulting stream of intensity modulated, digital data is input to a computer programmed to process the data to reveal and record defect related data. Through data compression techniques, the scanned lines may be displayed in slow motion on the video monitor. In data compression, normal transmitted or reflected light intensity signal values may be dropped out, highlighting signal values which correspond to a defect. Since defects are typically introduced by up-stream processing or transporting of the web, the defects are typically repetitive and appear as a vertical line on the display. Thus an operator is alerted to surface defects as they begin to appear and can make real time corrections while the web is moving or can shut down the web to inspect the equipment. At the same time, the defect data is stored in memory in relation to stored position values picked up as the web is moved through the scanning station so that the web may be visually inspected in those locations.
Such illuminators often have a length corresponding to the width of the web to be inspected. For example, illuminators used in paper and sheet metal fabrication are often seven feet long and eight inches square in cross-section and employ long incandescent or fluorescent lamps. Fluorescent lamps are not intrinsically bright, and it often becomes necessary to drive the lamps hard electrically, over their ratings, and place them fairly close to the web. Bright illumination is necessary because imperfections are often small, of low modulation, move past the inspection station rapidly and occasionally only detectible under special illuminating conditions. Such illuminators get quite hot, and the heat changes the glass pressure within the fluorescent lamp, affecting light output and tends to damage plastic optical components. Cooling of such illuminators is usually difficult in the restricted confines at the web inspection station and because of the small cross-section and long length of the illuminator.
Another method of inspection requires a polygon-shaped mirror or holographic scanning system to scan a laser beam across the web. Rotating mirror laser beam scanners require high precision, motor driven rotating mirror facets and optical systems for scanning the laser light beam accurately across the web with equal intensity of the beam reaching the extreme positions of scan in the relatively flat scanning angle. Aside from safety precautions inherent with the use of a high intensity laser beam and imperfections in the facets of polygon shaped mirrors, laser beams do not provide the white light source which is useful in identifying imperfections of differing color on or in the web being scanned.
Various cylindrical, linear light sources have been disclosed in the prior art for use in web inspection and also for use in telecine and the radiographic image scanning. In such scanners, it is common to provide an illumination system which produces a line of light across the film. The light source in such systems produces a circularly symmetric light beam, and a problem in these systems is to provide for an efficient conversion of the circular beam to a uniform line distribution of the light. U.S. Pat. No. 4,868,383 assigned to the assignee of the present application discloses a linear light source for a film scanner that includes an elongated cylindrical integrating cavity having diffusive reflective walls and an input port through which an intense beam of light is introduced into the cavity. An output slit parallel to the long axis of the cylindrical integrating cavity emits a line of light which may be transmitted by or reflected from moving film. Light transmitted by or reflected from the film is imaged onto a CCD linear array or a photo sensor.
Other linear light sources are employed in plain paper photocopying and other applications where the line of light produced by the integrating cavity is highly diffuse so that artifacts on the media being scanned, such as scratches, fingerprints or the like, will not appear in the image produced from the scanned information. Thus, in systems of that type it is highly desirable to avoid detecting surface defects and irregularities while providing sufficient light of constant intensity along the length of the line of light to be modulated by the varying densities of the images in degrees allowing differentiation of the image's gradation by the CCD or photo sensor. Such a linear light source for a photocopier is illustrated in U.S. Pat. No. 3,947,106.
The environments in which these scanning systems are used are relatively clean. By contrast, industrial web inspection is often conducted under hot and dirty conditions with either dust or oil mist in the air which accumulates on the scanning equipment. Such accumulated dust or mist attenuates the light output and the heat accentuates the difficulty in cooling the high intensity light source.
It is preferable to employ a fixed, linear light source which provides an elongated light beam that is tailored to provide equal intensity light output alone its length and where the light is narrowly focused onto the web. Such constantly on light sources take advantage of the exposure accumulation properties of CCD elements.